1. Field of the Invention
This invention relates generally to inflatable type modular occupant restraint systems for passenger vehicles or, as they are more commonly known, air bag restraint systems. Such a system may be installed in an automobile or other vehicle, at least in part on the steering wheel for the protection of the driver and also in the dashboard or passenger side instrument panel for passenger protection in the event of a collision. More particularly, this invention relates to an improved filter unit for use in passenger side air bag restraint systems and a method for manufacturing the filter unit.
2. Description of Related Art
The air bag restraint system is located forward of the occupants and normally is in a deflated condition. In the event of a crash, however, the air bag is inflated in a matter of 20 to 40 milliseconds by a gas produced by the burning of a gas generant material in an inflator. Common features of such inflators that are used for the inflation of air bag cushion restraint systems are the inclusion in a housing of a gas generant composition containing a pelletized alkali metal azide contained in a basket and a filter unit external to the basket to cool the gas positioned between the basket and gas discharge orifices, as defined by the housing. The gas generant is ignited by an igniter. The gas generant burns rapidly usually with a temperature of about 1200.degree. C., and develops pressure in the housing in excess of 2000 psia. Further, the gas that is generated contains combustion residue including fine molten particles of metal and/or reactant oxides. The pressure of combustion gases resulting from the rapid burning of the gas generant material causes gas to rush into the bag to effect a rapid rate of inflation thereof. The cooling and filtration of the gas to remove the fine particles for the safety and comfort of the passengers is of great importance.
Filter units typically available in the art include alternating layers of screen and ceramic paper. For example, Paxton et al., U.S. Pat. No. 4,998,751, which patent is assigned to the assignee of the present invention, disclose a filter which includes two wraps of nickel coated carbon or stainless steel 30 mesh screen, then metal filter such as 80.times.700 or 50.times.250 mesh stainless steel or 40.times.180 mesh nickel coated carbon steel, then a single wrap of ceramic filter paper 0.080 inches thick, followed by two wraps of 30 mesh stainless steel or nickel coated carbon steel. A wrap of 5 mesh stainless steel or nickel coated carbon may be provided adjacent the wall of the tubular portion to allow space around apertures therein for the free passage of generated gases.
The filter units are, typically, manufactured by rolling the different filter materials circumferentially around a mandrel similar to rolling wrapping paper circumferentially around a core. For example, Cunningham, U.S. Pat. No. 4,878,690, which patent is assigned to the assignee of the present invention, discloses a filter unit of this type of manufacture. The filter units are manufactured as single units by hand and with the aid of a machine. The geometry of the filter unit is determined by the vehicle into which the filter unit will be installed.
The individual filter elements for each filter unit are cut to size and are placed by hand on the mandrel of an automatic wrapping machine which wraps the filter and spot welds the outer layer. The filter unit then is removed from the wrapping machine by hand and the cycle is repeated for the next filter unit. The cutting of each piece of filter material and the hand placement of each piece of filter material on the wrapping machine significantly increases the time of manufacture, (cycle time), for each filter unit. This manufacturing system is incompatible with the high volume production requirements necessary to meet the demands of the automotive industry.
The consistent and economical manufacture of filter elements for passenger side gas generators which will efficiently cool and clean gas from the gas generant is of great concern. This must be done in light of the fact that the size of the passenger side generator, and therefore the filter element, is strongly dependent on the geometry of the automobile's interior and therefore varies significantly from automobile to automobile.
The use of spiral wrapping is disclosed in Merrill et al., U.S. Pat. No. 4,101,423, for microporous layer over a porous inner layer for filtering bacteria from fluids. Shevlin, U.S. Pat. No. 4,398,931, discloses spiral wrapping a woven ceramic filter material over a rigid tubular cage for radially-inward filtration of hot industrial gases. The Shevlin invention is designed for a pressure drop of from 2 to 6 inches of water at a flow rate of 3.2 to 24.2 per minute per dm.sup.2 of filter area.